426 Chronicles of Science. [July, 



gradually by a continuous change, and yet be made of rigid and 

 unchangeable material, might seem at first sight to be an impossi- 

 bility ; but, after all, when the result is accomplished, as in this 

 apparatus, we are surprised as much by the simplicity as by the 

 ingenuity of the means employed. The woodcuts almost explain 

 the apparatus of themselves ; but we may say, in addition, that it 

 consists of two cylinders or rollers with parallel axes and surfaces 

 in contact, having similar conical grooves on their surfaces, and 

 fine teeth cut at one end of each, which, gearing together, cause 

 them to rotate in unison. There is, theoretically, an objection to a 

 diaphragm of this construction, from the fact that its opening will 

 not always be in the same plane — that is, the smallest cross-section 

 of the space between the rollers will not always be equidistant from 

 a plane at right angles to the line of sight and passing through the 

 axes of the rollers. With the larger opening, this cross-section will 

 be nearest to, and with the smaller, farther from, such a plane. In 

 practice, however, this difference is so small as to be entirely unim- 

 portant, and may even, in some cases, be turned to advantage. 



Experiments have been made at Toulon by M. F. Silvas to try 

 to attach to life-buoys another floating body provided with phos- 

 phide of calcium, which, on becoming wet, gives off spontaneously 

 combustible phosphuretted hydrogen, thus emitting light to guide 

 the man, who might have fallen overboard and be in search of the 

 life-buoy. 



Heat. — Dr. Guy has arranged in series the different poisonous 

 substances according to their melting and sublimation temperatures. 

 The arrangement is as follows : — (1) Sublimates formed without 

 any previous change of state of aggregation, and giving white 

 vapours ; under this head are brought bichloride of mercury, calo- 

 mel, arsenious acid, and cantharidine. (2) Sublimates after pre- 

 vious fusion, and without leaving any residue — viz. oxalic acid. 

 (3) Sublimates after previous fusion, leaving a carbonaceous residue 

 — morphine and strychnine. (4) Fusion, change of colour, subli- 

 mation and deposition of carbonaceous residue, aconitine, atropine, 

 delphine, veratrine, brucine, digitaline, picrotoxine, solanine. (5) De- 

 crepitation ; slow and partial sublimation ; tartar emetic. 



Professor Morren has instituted some experiments on the com- 

 bustibility of diamonds, and the effect of a high temperature on 

 these gems. The author, in a letter, first rektes the following facts 

 as having given rise to his experiments. A jeweller at Marseilles 

 was requested to enamel afresh the gold bearings of two large 

 diamonds of great value, used as shirt buttons. Instead of taking 

 off the diamonds, always a delicate operation, the jeweller, who had 

 frequently executed such work previously, decided to enamel the 

 gold while the diamonds were left on their bearings. Not having 



